I agree with SanDiego1K that for the amount of time you have, it would be more profitable to just stay in BsAs and skip Montevideo. Although I like Montevideo, there's really not that much to do and overall BsAs is a much, much more exciting and interesting place. However, if you go back another time and have longer to visit, try Punta del Este (the Uruguayan beach resort about 2 hours from Montevideo). To me, Punta is at its best in the early part of the "off season" (the 2 - 3 weeks immediately following Easter) when the crowds have left, but the weather is still warm enough to swim.